In recent years, in terms of economy and environmental considerations, fuel efficient tires have been required. As a means for obtaining such a fuel efficient tire, reducing the rolling resistance of the tire has been of interest.
Here, as a method of obtaining the fuel efficient tire by effectively reducing the rolling resistance, it has been proposed to reduce hysteresis loss which is a major cause of the rolling resistance. Specifically, as a method of obtaining the fuel efficient tire, it has been proposed to reduce the hysteresis loss (energy loss sometimes referred to also as “strain energy loss”) caused for example by a change in a case shape of the tire during running.
As the fuel efficient tire that reduces rolling resistance by suppressing generation of the hysteresis loss caused by the change in the case shape of the tire, there has been suggested a tire having a carcass cross-sectional shape in a tire width direction having a non-natural equilibrium structure (for example, see Patent Literatures 1 and 2). More specifically, Patent Literatures 1 and 2, as the fuel efficient tire with reduced rolling resistance, disclose a pneumatic tire that, in a tire width direction cross-sectional view, has a curvature radius of the carcass positioned in a tire radial direction outer portion of a sidewall portion that is smaller than the curvature radius of the carcass in the natural equilibrium shape in the same position. According to the pneumatic tire described in Patent Literatures 1 and 2, by reducing the curvature radius in the tire radial direction outer portion of the sidewall portion, where energy loss under shear deformation is great but energy loss under bending deformation is small, reduces bending deformation during running in the tire radial direction inner portion of the sidewall portion where the energy loss under bending deformation is large (a portion close to a bead portion) so as to suppress generation of the hysteresis loss, thereby reducing the rolling resistance.
Here, the “natural equilibrium shape” means a shape of the carcass of a tire derived with reference to a known natural equilibrium shape theory, when the tire is mounted on an applicable rim and applying an internal pressure corresponding to a maximum load capacity defined by standards such as JATMA and the like, and generating no shearing strain and bending strain except a strain caused by tensile force on the carcass cord.